Dynamo woes

Hi folks,
Despite owning my ‘56 Airone for a few years now I have not connected up the dynamo until recently. After starting the bike today I tested the lights ( now converted to led ) and was surprised to see them flickering badly. They are both bright, so maybe flickering isn’t the best word, but a very fast strobe effect ( a bright flicker ? )
Does this indicate a faulty dynamo ? I also suspect the regulator, which must be built into the dynamo as I don’t have a separate item anywhere.

How can I test it ?

I would have thought the battery should smooth out any pulsing from the dynamo.
Have a read of this article in the Google Guzzi singles group (hopefully the link will work for you)
https://groups.google.com/g/guzzi-singles/c/kcqoRm-r5rY/m/2XAHoanFAgAJ

Thanks Don. That’s an interesting thread. Nothing specifically about my flashing lights issue but it has given me something to go on. It also occurred to me that it might be a problem with the led light bulbs, so I swapped these back and the problem remains ( albeit a lot dimmer ).

I’ll update if/when I find a fix.

Hopefully that article will give you an idea of how the dynamo works, the various connections etc and how to test yours.
It is well worth joining the group as there are a lot of knowledgeable people who contribute.

Yes, I am also in the Guzzi Singles Google group, but I like to come here first. I did a search in their history for other dynamo issues and found very little beyond that thread.
I measured the output from the dynamo and I only get 6.2 volts, at best, from it. I think it should do better if it is to keep the battery charged when the lights are on. Searching the web, I can find plenty of info about Lucas dynamos and how to test them, but weirdly very little of this seems to apply to the Marelli dynamo. I would have thought that they all worked on the same principles but it appears not.
My suspicion is that the led bulbs are flickering because of the way that they work. That is, they need a full 6v in order to light up, whereas an incandescent bulb will still glow. Thus, if the dynamo is giving an inconsistent output, the led bulb can’t keep up.

I agree with your comment that the battery should even out any pulsing from the dynamo, so maybe the problem is with my battery.

I’ll be back.

I presume the 6.2V is the regulated output. The link I gave does mention adjusting it to vary the output voltage. I would be tempted to tweak it up a bit. Not sure what the recommended voltage is for charging a 6V battery. 12V ones usually charge at 13.8V.
I presume your LED lamps are rated for 6V operation?

Well . . . I have managed to learn more about the lighting and the charging circuit. First off; yes the led lights are rated at 6v, but as I suspected they will only work if there is at least 5.9 volts getting to them. Otherwise, nothing. Secondly; my dynamo will only give 6.2v at best. This means that the voltage in the battery is hovering around 6v which results in the flickering. I should also add that the dynamo itself takes a lot of current. For example, the live wire to the brake light switch shows 6v with the dynamo disconnected, but only 3.5v when the dynamo is connected.


The battery I currently have on the bike is a little 4Ah lead acid battery. I bought this as it fits neatly into a rubber battery box which looks correct for a 50’s bike. As a test, I wired up a 6v 11Ah battery from another bike. Using this battery the lights no longer flicker. However, I suspect that if I kept the lights on long enough, it’s voltage would drop below 6v and I would have the flickering again.

So, I think the real problem is that the dynamo is not producing sufficient charge to keep the battery above 6v. I have found a small screw on top of the dynamo which - appears to - affect the output, but I only see a difference of about 0.4 volts turning it in both directions. Screw is shown in the picture below

Does this dynamo need to go to the doctor ?
Airone dynamo.jpg